1. Field of the Invention
This application is related in subject matter to copending application Ser. No. 07/448,718 filed Dec. 11, 1989, now U.S. Pat. No. 5,042,937, for "Pupil Function Analyzer." application Ser. No. 07/448,718 is assigned to the assignee of this application and is incorporated herein by reference.
The present invention generally relates to automated instruments for use in examination of a patient's vision, and more particularly to an improved clinical method and apparatus for identifying and measuring regions of reduced sensitivity in a patient's visual field by means of pupillary light response.
2. Description of the Prior Art
Perimetry, the identification and measurement of regions of reduced sensitivity in the visual field of an individual, is of fundamental importance in the diagnosis of a wide variety of disorders of the visual system. It is performed on a significant proportion of all patients seen by ophthalmologists. There are two types of perimetry visual field testing commonly used in the prior art; a kinetic method and a static method. Both require a judgmental response on the part of the patient.
In the kinetic method, a light spot or other suitable stimulus of a known size and brightness is moved inwardly from beyond the edge of the patient's peripheral vision until the subject signals in some manner that he or she sees the stimulus. In the static method a stationary stimulus is displayed successively at a series of points and the patient reports whether or not the stimulus is seen. If not, the brightness of the stimulus may be increased in steps with intervening pauses at each point until the patient signals that he or she sees the stimulus.
Sometimes these prior art procedures are automated and sometimes performed by hand. In either case, this procedure confronts the patient with the following task. The patient is asked to keep looking at a target straight ahead and report whether or not he or she sees other spots of light presented at different places in the visual field. Even if the task were to report whether or not a spot is visible where the patient is looking, many patients would have a difficult time making each judgment. Trying to make a judgment about seeing a spot in a location where one is careful not to look is much more difficult. The result is that for many patients their response is not reliable and the examination does not provide a good indicator of pathology.
There also have been proposals in the prior art to perform perimetry with apparatus which does not require the patient to make judgments; for example, U.S. Pat. Nos. 3,883,235 and 3,718,386. One of these proposals is to monitor pupil size as a response to the stimulus. In this procedure spots of light flash at various places in the patient's visual field while his or her pupil size is monitored electronically. A detectable change in pupil size correlated in time with a flash indicates that the visual system has detected the flash. This prior art pupillary response perimetry system is essentially the same as the widely used procedures requiring judgmental patient response except the response is pupillary rather than requiring the patient's judgment. These prior art proposals for perimetry systems using pupillary light response are not altogether practical for clinical application. The pupillary system is somewhat sluggish. After a latency of about 200 milliseconds, the response to even a brief flash of light lasts for a few seconds; the response time is a function of the intensity of the flash and the sensitivity of the device which measures pupil size. These prior art procedures can obtain measurements at the rate of only one each one or two seconds, which is comparable to the rate at which measurements can be obtained using a patient's psychophysical judgment. If pupil responses could be reliably detected, prior art pupil responsive perimetry would thus take about as long as the commonly used perimetry methods requiring patient response. However, a test spot must be fairly small if the map of the patient's field of view is to have sufficient resolution to be useful, and the pupillary response to a small spot is small. Because a pupil continuously undergoes apparently random variations in size, even in the absence of light, the small responses to flashed spots are difficult to detect reliably. To produce pupil responsive perimetry maps with prior art techniques, each location should be tested with a number of repetitions, resulting in an impractically long procedure.